Corrosion inhibitors



Patented June 20, 1950 UNHTE 2,5l2,59 FFHQE rear Glenbrook, Conn., assign ors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application March 31, 1942,

Serial No. 436,976

1 Claim.

This invention relates to the prevention of corrosion in processes wherein cyanamide or its dimer, dicyandiamide, is converted to other organic compounds by processes wherein these materials are heated at high temperatures and pressures in the presence of solvents containing ammonia. More particularly, it embraces the production of such products in a condition substantially free from contamination by iron, nickel or similar metallic impurities by carrying out the conversion in stainless steel autoclaves and in the presence of inhibitors such as phosphates, chromates, and the like.

It is well known that cyanamide and dicyandiamide are important raw materials for the production of a great number of organic nitrogen compounds. For example, melamine can readily be prepared by reacting a charge consisting of 1 part of cyanamide or dicyandiamide, l to 2 parts of anhydrous ammonia and 1 to 2 parts of methyl alcohol or a similar organic solvent at a temperature of l-200 0., over a period of 4 to 12 hours. Various modifications of this process result in the production of guanidine salts, as by introducing equivalent quantities of the corresponding am monium salts into the reaction mixture.

Heretofore, it has been found that when such reactions are carried out in autoclaves made from alloys such as those known commercially as highchrome or stainless-steels, the products are contaminated with an excessive amount of undesirable impurities since the autoclaves, fabricated from such iron alloys with about 8-20% of chromium content based on the total weight of the alloys, although somewhat resistant to the attack of such mixtures are nevertheless slightly corroded. This results in products containing undesirable quantities of iron, nickel and similar impurities which are derived directly from the alloys used in the autoclaves.

It is an object of the present invention to provide a method for protecting these mixtures and the autoclaves wherein they are reacted from the corrosive effects of the conversion prodnets of cyanamide, dicyandiamide and similar organic compounds. It is a further object of this invention to provide a method for the protection. of chrome-alloy or chrome-nickel alloy steel apparatus used in handling and processing cyanamide, dicyandiamide, guanidine nitrate and similar compounds. A further object of the products of cyanamide, dicyandiamide, guanidine nitrate, and the like, with a, liquid containing diammonium phosphate, ammonium dichromate, sodium dichromate, and similar corrosion inhibitors. These inhibitors are preferably present in concentrations of from 50 to 1000 parts per million, based on the total autoclave charge.

It is well known that ordinary ammonia, whether aqueous oranhydrous, is not corrosive to unalloyed iron or steel at temperatures extending up to 400 C. It is also known that neither cyanamide nor dicyandiamide attack iron or steel when these compounds are stored in containers or shipped in such containers. Solutions of cyanamlde or dicyandiamide in liquid ammonia, whether treated with methyl, ethyl or similar alcohols, have likewise been found to be non-corrosive. However, when such solutions of cyanamide and dicyandiamide in liquid ammonia are heated to temperatures whereat they are converted to such products as melamine, guanidine nitrate, and the like, it has been found that ordinary steel autoclaves and even autoclaves ties as iron, nickel, chromium, and the like.

present invention is to improve the above and similar conversion processes by conducting them in iron and chromium alloy autoclaves in the presence of corrosion inhibitors such as diam- I monium phosphate, ammonium dichromate, sodium dichromate, and the like, in order to obtain products that are substantially free from contamination by iron, nickel, and the like.

Broadly speaking, the invention embraces bathingor wetting, continuously or intermittently, the metallic surfaces exposed to the conversion This rapid attack by cyanamide and dicyandiamide solutions in liquid ammonia during conversion upon the walls of an autoclave assumes very serious proportions, since ferrous equipment is the only commercially available material that will withstand the great pressures involved in this type of conversion. For example, in the process for the manufacture of melamine, autoclave pressures of 3000 pounds per square inch are not uncommon and similar pressures are also necessary for the production of other guanidine salts. If non-ferrous equipment were to be used, an autoclave capable of withstanding such pressures safely would be extremely expensive to design andmaintain if commercial batches were to be produced.

In accordance with one embodiment of this invention, it has been found that a class of ferrous alloys known as high-chrome steels, alloys of iron containing about 8-20% of chromium based on the total weight of the alloys and stainless steel, are capable of withstanding the pressures above mentioned with a large factor of safety, and can be used to fabricate autoclaves. When from to 500 or more parts per million of inhibitors such as ammonium phosphate, ammonium dichromate, sodium dichromate, and the like, are added to the reaction mixture in such autoclaves, products containing very little, if any, ferrous and similar impurities are obtained. The above high-chromium steel alloys are usually referred to as low carbon steels, since they contain from about .06% to about .2% of carbon, and in addition varying proportions of other elements such as smaller percentages of sulfur, selenium, and silicon, and not more than about 15% of nickel. Such alloys as contain nickel are preferably stabilized by the addition of molybdenum, columbium, titanium, manganese, or mixtures of two or more of such elements up to approximately ten times the amount of carbon present. Usually from 8-12% of nickel is present in these alloys. This improves their machining properties and facilitates their fabrication. However, more than 15% of nickel tends to increase corrosion in the alloys.

The conversions of cyanamide or dicyandiamide embraced by this invention are effected in mixtures containing liquid ammonia in the autoclaves above described and preferably at high temperatures and pressures. Usually the autoclaves are fabricated from or lined with the above alloys. The reactions are carried out without corrosion of the alloy liner or autoclave, and without excessive contamination of the product with iron, nickel or other metallic impurities contained in the alloy, by adding effective amounts, of the corrosion inhibitors given in more detail hereafter. The addition of such corrosion inhibitors results in a remarkable resistance on the part of the alloy to corrosion during the reaction. The purity of the products obtained is further illustrated by the following examples, which, however, are not intended to limit the invention except as further defined in the claims.

In each of the following examples, an autoclave fabricated from a high-chrome steel or fitted with a liner made of a high-chrome steel was charged with a mixture containing 450 g. of liquid ammonia, 473 g. of ammonium nitrate containing at most 12 parts per million of iron and 250 g. of dicyandiamide containing parts per million of iron. This reaction mixture was During the remaining runs the liner lost 94 grams in weight. This clearly indicated that corrosion of the liner contributed to the greater part of the contamination of the product.

5 In order to see whether the use of iron or nickel alloys stabilized with molybdenum or columbium which were given a metallo-graphic finish by pickling them in a nitric acid-hydrofluoric acid bath and subsequently passivating them in a warm 20% nitric acid bath, liners'having the composition shown in Table II were used to prepare melamine using the following charge:

Equal parts by weight of dlcyandiamide, liquid ammonia and commercial methanol were heated in the autoclave reaching a temperature of 180 C. over a period of one hour. The charge was held at 180-190 C. for two more hours after which it was cooled to room temperature and the contents removed. The products contained on an average of 0.034% of iron.

' TABLE II Per Cent Composition (balance iron) Logs or Example Weigh t,

' 0 Cr Ni Si Mn Mo Co grams The material produced in each of the various runs is thus seen to be unduly contaminated with iron, nickel and similar impurities. Because of this, the following runs were made using the inhibitors indicated with the reaction mixture used in the examples of Table I.

TABLE III Ammonium dichromate inhibitor heated to 160 C. over a period of about one hour and held at 160 C. for an additional hour where- Parts per Million upon it was cooled and a pure white guanidine Example Product nitrate obtained in those cases where an inhibitor Inhibitor Iron Nickel Chromium was used in effective amounts.

1.000 46 4 512 Slight ink. TABLE I 1.000 16 1 550 DJ. 186% 11 2 Wh Parts per Million n 27 Example Inhibitor Chromium Product Iron Nickel TABLE IV Diammom'um phosphate inhibitor 1.030 Very dark. 2.130 1),. 2, 910 D0. Parts per Million 1.330 Do.

go. Example I h b Ch Product O. F n l YO- 560 Do. itor Iron Nickel mium 2,140 Do. 930 no. 1,120 Do. 18 1 1 Excellent, white. 3,800 Do. 15 1 1 Do. 12 1 1 Do. 13 1 1 Do. In the first three runs the liner of the auto- 17 1 1 clave lost 23.5 grams during the experiments.

TABLE V Diammonium phosphate inhibitor Parts per Million Example Product Inhibitor Iron Nickel Chromium 24 2,000 14 2 1 Excellent, white. 25. 1,000 14 1 1 Do. 26......... 500 13 1 1 Do. None, liner of 26 13 1 1 Do.

used. 28 Same as27 16 1 1 Do. do 20 l 2 Do.

This series of experiments showed that by using an inhibitor, a passivated film is formed on the liner which retains its effectiveness for several runs thereafter.

TABLE VI tions can effectively be used to prevent corrosion of the autoclave and contamination of the product when used to prepare guanidine carbonate from reacting proportions of dicyandiamide, am

monium carbonate and water in the presence of Diammonium phosphate inhibitor hquld ammonia- The examples herein given clearly show that ordinary high-chrome steels are not in them- Parts i Minion selves satisfactory corrosion resistant materials Emma mm Pmdm for reactions wherein cyanamide or dicyants: Nickel chmmmm diamide is a reagent. Moreover, it is to be understood that various other chromate or chromate- 3 11, 2 4,202 gi g gfiion-containing compounds such as various other 32.....- 125 4 2 Do. 16 alkali-metal chromates, namely potassium chr0- mate and bichromate among still other chromate In order to determine the minimum amount metallic salts, can be used and are to be COIlSidof inhibitor necessary for efiective prevention of ered as embraced within the broad limits of this corrosion, the following experiments were carried invention. out in accordance with previous autoclave charges. It is to be understood that the examples given TABLEVII Diammonium phosphate inhibitor ParisperMillion Example Product Inhibitor Iron Nickel Chromium as N new 1,000 3,000 Very dark. 34 12,200 1, 510 4,000 Rather dark. 35 60 a1 4 1 Excellent, white. as so so 2 4 Do. 21 7s 2s 1 2 Do. as 1 11 1 2 Do. 39--.... Nonehliner oi 38 25 2 3 Do. 40. Salt as a9. 8, 310 400 2, 1100 Rather dark.

From these experiments it is seen that at least in detail vare merely illustrative and not limita- 50 or more parts per million of the inhibitor tive embodiments of the invention, the scope of are necessary to prevent contamination of the which is more clearly indicated in the following product. Moreover, with the small amount of claim. inhibitor used, although the liner of Example 39 40 We claim: was still effectively passivated, the next run In the conversion of a compound selected from (Example 40) showed that only a very thin prothe group consisting of cyanamid'e and dicyantecting layer was present (compare Table V). diamide to a derivative thereof, and including TABLE VH1 the step which consists in heating said compound while dispersed in a solvent comprising liquid Ammonium chromate inhibitor ammonia in a closed pressure vessel in which all the internal surfaces in contact with the reaction Partspermillion mixture consist of a ferrous alloy selected from Emlfle @mduct the group consisting of high-chrome and stain- Inhib- Imn Nickel .Chroless steel, the improvement which consists in pmtecting the metallic surfaces exposed to the reac- 7 tion mixture by, adding from 50 to 1000 parts of 3:: 82 3?, 3 gfg gflg e ammonium phosphate per million parts of the 43 2.50 21 Do. reactants. ROBERT c. swam.

JOSEPH H. PADEN.

Since iron was the principal contaminating material, these examples were analyzed for the presence of 11-011 only, REFERENCES CITED TABLE 12: 00 The following references are ,of record in th sodium dichrom'ate inhibitor, me this patent- UNITED STATES PATEN'IS Number Name Date Ex ple ma 4 Product 5 1,841,825 Kriegsheim Jan. 19, 1932 nn N w 3?, 1,875,982 Boiler Sept. 0, 1932 1,988,584 Dana Jan. 22, 1935 45 N 1,390 1 0 330 slightly co]md 2,056,283 Lawrence 001i- 6, 1936 g s f i t wine 2,129,689 Hetherinaton Sept. 13,1988 48" 100 18 2 7 D3; 2,191,361 Widmer- Feb 20, 1940 21 V 1 1 2,273,781 Hochwalt Feb. 1'7, 1943 In a. manner similar to those given above-for the production of melamine and guanidine ni- O'I'HERREFERENCES Snell: Industrial and Ensineerlng Chemistry,

mate, the same inhibi ors. in the same ptopor- 7 vol. 33. No. 6, page s74, my 1931,, 

